JPH04318432A - Distribution temperature measuring method by optical fiber sensor - Google Patents

Abstract

PURPOSE:To obtain a temperature data of high measuring accuracy by a method whereby two or more measuring signals are obtained for the same point by means of an optical fiber sensor, converted to electric signals thereby to obtain the temperature data while the influences upon the two or more measuring signals because of the difference of the transmission loss, peripheral temperature and temperature change of a light source are corrected with the utilization of symmetry of the temperature data. CONSTITUTION:An optical fiber of two cores is bent thereby to form an optical fiber sensor 1. Two measuring signals for one point of the sensor are measured by a distribution temperature measuring part 10, converted into electric signals at photodiodes 16, 16', and sent out of an interface 19. In a processor part 20, a correcting/processing part 25 corrects the measuring signal thereby to obtain a signal from which influences of various kinds of factors are removed, and a temperature data computing part 26 obtains the temperature data based on the signal.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of measuring distributed temperature using an optical fiber sensor in a temperature measuring section.

0002

[Prior Art] For example, in the ocean, the temperature (
When measuring the distribution temperature (hereinafter referred to as distributed temperature), the conventional method using thermocouples requires a huge number of thermocouples, which is disadvantageous if the distance is long. Therefore, a method of measuring distributed temperature using an optical fiber sensor has recently become popular. The outline of this measurement method is as follows. For example, a laser diode is driven to send out an optical pulse signal of a reference wavelength, which enters the input end of an optical fiber sensor.
The backscattered light that is scattered at each part in the length direction of the optical fiber and returns to the input end is extracted by an optical directional coupler.
The Stokes light and anti-Stokes light of the Raman scattered light are extracted from the light through two different wavelength filters and converted into electrical signals by a light receiving element, and the time distribution of the received light signals is distributed for each position in the length direction of the optical fiber. The temperature is calculated and measured from the received light power of the signal based on a predetermined arithmetic expression. In this case, the optical fiber sensor used is a single optical fiber.

0003

[Problems to be Solved by the Invention] By the way, the above-mentioned method of measuring distributed temperature using an optical fiber sensor is superior to the conventional thermocouple method in that it can measure distribution over a wide range at low cost. . However, at present, measurement methods using optical fiber sensors generally have lower measurement accuracy than conventional thermocouple methods. The measurement distance is approximately several kilometers, the distance resolution is several meters, and the measurement accuracy is ±1°C.

[0004] In the conventional measurement method using an optical fiber sensor, since the optical fiber is a single optical fiber, there is only one type of measurement data at a given length position, which lacks reliability. For this reason, in order to obtain measurement accuracy, the received light signal is converted into an electrical signal, which is then repeatedly added and averaged at the same point within a predetermined sampling time to increase the S/N ratio. This signal is averaged over a longer period of time to determine the temperature distribution using a predetermined calculation formula.

However, since this data processing method simply performs addition and averaging, there is a limit as measurement accuracy cannot be obtained beyond a certain amount of addition processing. In addition to the above problems, the temperature measurement data of the optical fiber sensor has a difference in transmission loss due to the wavelength dependence of the optical fiber's transmission loss, that is, the difference in wavelength between Stokes light and anti-Stokes light (about 80 mm). . For this reason, even if a uniform temperature distribution area is measured, a greatly tilted temperature distribution may be observed. This propagation loss difference also changes depending on the ambient temperature. Further, a similar distribution may be exhibited depending on the temperature change of the laser light source.

[0006] Conventionally, therefore, it has been necessary to measure the temperatures near both ends of the optical fiber sensor using separate reference measurement means, and use this reference data to correct the measured data during subsequent analysis. Therefore, at least two or more sets of thermometers are required as a reference temperature measurement means for correction, and measurement at two or more different points requires time and effort. In addition, for example, if part of the temperature measurement data shows an extreme value, it is difficult to distinguish whether it is due to a sensor abnormality or the true temperature, and it is difficult to judge when analyzing the temperature measurement data later. There was also the problem of

The present invention was made in view of the various problems associated with the conventional method of measuring distributed temperature using an optical fiber sensor as described above. Errors in temperature measurement data that occur during measurement due to various factors when obtaining temperature measurement data from optical signals measured using optical fiber sensors that obtain signals are minimized by simultaneously adding processing to correct the effects of those factors. It is an object of the present invention to provide a method for measuring distributed temperature that can provide temperature measurement data with higher measurement accuracy.

[0008]

[Means for Solving the Problems] In order to solve the above-mentioned problems, the present invention uses an optical fiber distribution temperature sensor consisting of at least two optical fibers folded at the length end to measure the temperature at any position of the sensor. Obtain optical signals from at least two locations representing the temperature distribution, convert these into electrical signals to measure the distributed temperature, and use the symmetry of the measurement data to measure the same point on the sensor with another reference temperature measurement means. This is a distributed temperature measurement method using an optical fiber sensor that corrects the measured data based on the reference temperature data obtained.

In the above measurement method, a light pulse signal of a reference wavelength is sent out from a light source and is incident on the input end of the optical fiber sensor, and the optical pulse signal is transmitted in the longitudinal direction of the optical fiber in order to obtain the optical signals at each of the at least two locations. The backscattered light that is scattered in each part and returns to the input end is extracted by an optical directional coupler, and the Stokes light and anti-Stokes light are respectively received by a light receiving element and converted into electrical signals. The distributed temperature may be measured by calculating the distributed temperature for each position in the length direction of the optical fiber, which is represented by the time of the signal, from the received light power of the signal based on a predetermined arithmetic expression.

0010

[Operation] The measurement principle of the above-mentioned distributed temperature measuring method according to the present invention is carried out as follows. As shown in Figure 7, when an optical pulse signal with a reference wavelength λo is input from a light source to the input end of an optical fiber sensor, each part through which the light passes is caused by the glass medium causing Rayleigh scattering, which is elastic scattering, and Raman scattering, which is inelastic scattering. Scattered light is generated. Raman scattering is caused by inelastic collisions between the molecular vibrations of the glass in the optical fiber and the passing optical pulses, and the Stokes light is shifted by the wavelength corresponding to the molecular vibration frequency with respect to the incident light of the standard wavelength to the longer wavelength side. (
+Δλ), and anti-Stokes light (−Δλ) is generated on the short wavelength side.

Of the scattered light, the light that returns to the input end of the optical fiber is called backscattered light, and since the intensity of Raman scattered light in particular changes due to the influence of temperature, this can be used to measure temperature. Do this.

Since the speed of light in the optical fiber is known, the time from pulse incidence to light detection represents the distance R between the location where the detected scattered light is generated and the incident end. Once the constituent material of the optical fiber and the wavelength of the incident light are determined, the ratio of the received light power of the Raman scattered Stokes light (λS = λO + Δλ) and the anti-Stokes light (λa = λO − Δλ) is calculated.
The temperature can be determined using the following equation. R=(c/2n)・Δt
(1) Ia/Is
=(λS/λa)4 exp(-(hcν)/kT
) (2) where, c: speed of light in vacuum, n: refractive index of optical fiber, Δt: time from pulse input to light detection, Ia: anti-Stokes light intensity, Is: Stokes light intensity, λa: anti-Stokes Light wavelength, λs: Stokes light wavelength, h: Planck's constant, k: Boltzmann's constant, ν: Raman shift amount, The above-mentioned backscattered light is separated and extracted by a light directional coupler in the detector, and the Raman scattering Stokes light and anti-Stokes light are extracted separately through filters with different wavelengths, but since their optical signals are extremely weak, sufficient averaging is performed to remove noise and the S/N ratio is increased during measurement. do.

When the temperature with respect to the distance R is calculated from the above equations (1) and (2), the temperature distribution in the longitudinal direction of the optical fiber is obtained as shown in FIG. In the measurement method according to the present invention, the distributed temperature data obtained in this way can be obtained by measuring the temperature at the same point at two locations on the optical fiber, since the optical fiber is folded back within the optical fiber sensor. The data is symmetrical around the folded end. Therefore, first, the position of the folded end can be accurately determined by utilizing the symmetry.

After accurately determining the length position of the optical fiber sensor, when calculating distributed temperature data corresponding to each position, correction is applied in advance based on separately measured reference temperature data. In that case, correction is made using the symmetry of the data so that the left and right data are symmetrical at the folding end. The above correction is based on the wavelength dependence of the transmission loss of the optical fiber, and applies an appropriate correction coefficient (2) to account for the difference in transmission loss, the influence of ambient temperature during measurement, or the influence of temperature change of the light source. This is done by multiplying the equation or adding a correction constant.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Examples of the present invention will be described below with reference to the drawings. FIG. 1 shows a schematic configuration of an optical fiber sensor 1 used in this embodiment. This sensor 1 consists of an optical fiber cable in which two optical fibers 1a sealed in a stainless steel tube 2 and five galvanized iron wires 3 are tied together. The optical fiber 1a is 50μ with a square refractive index distribution.
It is a multimode GI (Graded-Index) fiber consisting of a core of m and a cladding of 125 μm, the outside of which is coated with nylon, and the outer diameter is 0.6 mm. The stainless steel tube has an outer diameter of 1.7 mm (inner diameter 1.4 mm), and the gap between the optical fiber 1a and the stainless steel tube 2 is filled with a waterproof mixture. The galvanized iron wire 3 has an outer diameter of 1.6 mm, and the twisted optical fiber cable has an overall outer diameter of 5 mm. The optical fiber 1a is folded back at the folding end 4 of the sensor as shown, and the sensor 1 is constituted by the two-core optical fiber 1a.

The optical fiber sensor 1 is connected to a distributed temperature measuring section 10 via a connector 11, as shown in FIG. This distributed temperature measuring section 10 includes a control circuit 1
2 drives the LD drive unit 13 to generate the wavelength λo of the laser pulse signal from the semiconductor laser light source 13a.
The optical signal is passed through the filter 13b to the optical fiber sensor 1.
into the optical fiber 1a. The backscattered light generated in each part of the optical fiber is separated and taken out by an optical directional coupler 14, and another optical directional coupler 14' and two filters 15 (λa) and 15' (λs). The light is received by the two photodiodes 16 and 16' via the light receiving circuit 17, and converted into an electrical signal by the light receiving circuit 17.

This measurement signal is sent to addition memory 18 and stored therein. In this case, the backscattered light is extremely weak, so
A/D within a predetermined sampling time in the light receiving circuit 17.
After conversion, this process is repeated to perform addition and averaging at high speed, and the result is stored in the addition memory 18 and sent out via the interface 19. The measurement signal is converted into temperature data by the next arithmetic processing section 20 (controlling personal computer). This arithmetic processing unit 20 sends the measurement signal to the microcomputer 2 via an interface 21.
It is configured to send the data to 2 and perform arithmetic processing.

In the microcomputer 22, an averaging processing section 24 performs data averaging processing according to instructions from a central processing section (hereinafter referred to as CPU) 23. The averaging process in this case is performed by averaging measurement data obtained at the same point on the sensor at intervals of, for example, one minute, which are longer than the sampling time in the distributed temperature measuring section 10. Next, the correction processing section 25 applies correction to the measurement data. In this correction processing unit 25, for example, the temperature at a position near the incident end and the folded end of the optical fiber sensor is measured using another reference thermometer (thermocouple, etc.), and the value is input directly from the keyboard 30 or indirectly in the form of a correction index. The reference temperature value or correction index is used to provide a correction coefficient in advance when the next temperature data calculation section 26 calculates temperature data from the above-mentioned predetermined calculation formula. When the correction coefficient is given, the temperature data calculation section 26 obtains the required temperature data. The temperature data is displayed on a display 27, printed by a printer 28, and stored in a file 29.

The above correction method will be explained in more detail using FIGS. 3 and 4. First, when temperature is generally measured using a sensor without optical fiber folding, and the signal is converted into temperature data without the above correction, this temperature data has a Contains errors. When correcting temperature data including this error, the correction procedure includes three elements: (a) drift, (b) slope, and (c) magnification in FIG. 3. However, the graph in FIG. 3 is data measured in a thermostatic chamber in a laboratory, and the solid line in the figure is the temperature measured by the optical fiber sensor, and the dotted line is the true temperature. The central area is particularly cooled to create a rectangular graph. As you can see from this graph, in order to match the measured temperature data with the correct true temperature, true temperature information from at least three points is required. Since the temperature distribution graph is random and complicated, actual values from multiple points are required to improve accuracy.

However, since this embodiment uses a sensor consisting of a two-core optical fiber obtained by folding the aforementioned optical fiber, the symmetry of the obtained data is utilized, and the true temperature measurement information is obtained from the sensor input end and Correction is made as follows using the minimum true temperature data measured near the turning point. As an example, FIG. 4 schematically shows a graph assuming measured data of seawater temperature distribution (solid line) and the true temperature distribution (dotted line) using the optical fiber sensor 1 of this embodiment. The distance represents the depth of seawater. The true temperature points at the sensor input end position are P1 and P2, and the measurement point by the sensor is P1'
, P2'. If the temperatures of true temperature points P1 and P2 are T°C, P1' and P2' as shown in (a)
Adjust the temperature so that it reaches T first. This adjustment is performed for the temperature drift adjustment and slope adjustment described in FIG. Therefore, drift adjustment is performed by adding or subtracting a constant value, and tilt adjustment is performed by adding or subtracting a value that changes depending on the distance.

Next, as shown in (b), even after making the adjustment in (a), there is still a difference in the data around the turning edge.
Data is adjusted by giving a constant magnification as a coefficient around the folded end. At this time, in order to determine the magnification, data on the true temperature point P3 near the folded end is measured and provided by another means. In this way, temperature data as shown in FIG. 5 can be obtained by adding the above-mentioned graphical correction processing. Furthermore, when data is corrected by adding such a correction, data abnormalities as shown in Fig. 6 may occur, for example, due to (a) fusion loss at the connection point of the optical fiber, or (b) temperature abnormality. can be immediately determined (in (b), temperature abnormal points appear symmetrically).

[0022]

[Effects] As explained in detail above, the distributed temperature measurement method of the present invention uses two optical fiber sensors at the same point.
Measure the above measurement signals and use the symmetry of the data to add corrections to the measurement signals that take into account the factors that affect the measurement errors during measurement to remove as much error as possible during measurement. As a result, the measured temperature data is highly reliable and measurements can be made with extremely high accuracy.

[Brief explanation of drawings]

[Figure 1] Schematic configuration diagram of an optical fiber sensor according to an example

[Figure 2
] Overall schematic block diagram of the distributed temperature measuring device of the embodiment

[Figure 3] Explanatory diagram of three elements of correction included in the correction procedure

[Figure 4] Explanatory diagram of the actual correction procedure

[Figure 5] An example of measured temperature data

[Figure 6] Explanatory diagram of temperature abnormalities, etc.

[Figure 7] Diagram explaining the measurement principle

[Figure 8] Diagram explaining the measurement principle

[Explanation of symbols]

1 Optical fiber sensor 10　Distributed temperature measurement section 20 Arithmetic processing unit 25 Correction processing section 26 Temperature data calculation section

Claims (2)

[Claims] Claim 1: An optical fiber distribution temperature sensor consisting of at least two optical fibers folded back at its length end is used to obtain optical signals at at least two locations representing the temperature at arbitrary positions of the sensor, and these are converted into electrical signals. At that time, the symmetry of the measured data is used to correct the measured data based on the reference temperature data measured by another reference temperature measurement method at the same point on the sensor. A method for measuring distributed temperature using an optical fiber sensor, characterized in that: 2. A light pulse signal having a reference wavelength is transmitted from a light source and is incident on the input end of the optical fiber sensor, and at each portion in the length direction of the optical fiber in order to obtain each of the light signals at the at least two locations. Each backscattered light that is scattered and returns to the input end is extracted by an optical directional coupler, and the Stokes light and anti-Stokes light are received by a light receiving element and converted into electrical signals, and the time of the received light signal is Distribution by the optical fiber sensor according to claim 1, characterized in that the distribution temperature is measured by calculating the distribution temperature at each position in the length direction of the optical fiber represented by the received light power of the signal based on a predetermined calculation formula. Temperature measurement method.